1. Field of the Invention
The present invention relates to an ink jet print head having a plurality of ink ejection orifices capable of ejecting ink droplets and to an ink jet printing apparatus using the ink jet print head to perform printing.
2. Description of the Related Art
Printing apparatus are being used as an image outputting device in printers, copying machines and facsimiles, or as an image outputting device for composite electronic devices including computers and word processors and for workstations. Commonly known printing apparatus may be classified into an ink jet type, a wire dot type, a thermal type and a laser beam type. Of these, the ink jet type printing apparatus (ink jet printing apparatus), that performs printing by ejecting ink droplets from an ink jet print head onto a print medium, has many advantages over other types. The advantages of the ink jet printing apparatus include, for example, being able to form highly defined images easily and at high speed, to operate with a high level of quietness, to be constructed in small size and at low cost and to form color images easily. An ink jet print head used in the ink jet printing apparatus has a plurality of ink ejection elements formed therein at high density for faster printing speed and improved image quality. The ink ejection elements each comprise an ink ejection orifice formed in a front face of the print head, a liquid path communicating with the ink ejection orifice, and an electrothermal transducer (heater) installed in the liquid path. A large number of such ink ejection elements are arranged at high density. An ink jet printing apparatus that produces a color image generally has a plurality of such print heads.
The quality of images printed by the ink jet printing apparatus is greatly influenced by the construction of the ink jet print head (for example, the density of ink ejection elements). Thus, in addition to increasing the density of the ink ejection elements, as described above, various measures are currently being taken, for example, in the arrangement of ink ejection orifices (hereinafter merely referred to as orifices) and the volume of ink droplets ejected from the orifices. As one example, Japanese Patent Laid-Open No. 2003-127439 discloses an ink jet print head that can eject two kinds of ink droplets of different volumes from different orifices.
The ink jet print head disclosed in Japanese Patent Laid-Open No. 2003-127439 has a greater number of orifices for ejecting small-volume ink droplets than that of orifices for large-volume ink droplets. These orifices are arranged such that centers of orifices for small-volume ink droplets are located on imaginary lines running through centers of orifices for large-volume ink droplets in the direction of scan of the print head. This arrangement reduces density variations appearing as lines in printed images, assuring the printing of high quality images. That is, by setting the number of orifices for ejecting small-volume ink droplets greater than that of orifices for large-volume ink droplets, the image quality is improved in low-density (low gradation level) areas of the printed image.
Where an ink jet print head disclosed in Japanese Patent Laid-Open No. 2003-127439 is used, a faded image may be produced when high-density image areas fail to be printed at sufficiently high levels of density because the number of orifices for ejecting large-volume ink droplets is small. To prevent such image density reductions, the number of ink droplets applied to a unit area needs to be increased as by increasing the number of printing scans performed to complete a defined image area or reducing a speed at which to scan the print head. This makes it difficult to perform printing at high speed while keeping the high-density areas in good print quality. Further, to be able to perform the high-quality printing at high speed using the conventional ink jet print heads including the one disclosed in Japanese Patent Laid-Open No. 2003-127439, the number of orifices and the number of orifice arrays may be increased. This method, however, increases the size of a semiconductor board that integrates ink-ejection energy generation means (for example, ink-ejecting electrothermal transducers), giving rise to another problem of increased cost and size of the ink jet print head.